They give 2 very different sensations. A paddle tends to give sharper strikes (after you work up to it) and a flogger is generally used to work a person up slowly to a heightened state where their endorphines are pumping and they can take more. Like a prep.

If they are to be used on you then pick the one that you want to be used on you. If you are planning on using your new toy on others I'd recommend that yourself a good flogger. Not everyone is into paddles or being spanked. A flogger is a more versatile toy than a paddle.

As far as floggers go. Look for cowhide or suede. Suede is going to be softer and makes for a nice warm up flogger. If you go with cowhide you can also use it for a warm up flogger you will just have to vary how hard you swing it. Cowhide is a firmer leather and can provide more pain that the suede.

I have both a suede and cowhide flogger. I generally start with the suede flogger but depending on the person I am playing with I will start with the cowhide. Floggers are not just for warm up. I use a flogger pretty much the entire time I am having a scene with someone. Having a solid flogger can make a great bridge when stepping up to more stingy/painful toys.